Editing Skin

{{Short description|Soft outer covering organ of vertebrates}}
{{About|the skin of animals in general|skin in humans|Human skin}}
{{Other uses}}
{{Infobox anatomy
| Name        = Skin
| Latin       = cutis
| Image       = Elephant skin (3689577529).jpg
| Caption  = Elephant skin
| Precursor   =
| System      =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}

'''Skin''' is the layer of usually soft, flexible outer [[Tissue (biology)|tissue]] covering the body of a [[vertebrate]] animal, with three main functions: protection, regulation, and sensation.

Other [[cuticle|animal coverings]], such as the [[arthropod exoskeleton]], have different [[cellular differentiation|developmental origin]], structure and [[chemical composition]]. The adjective '''cutaneous''' means "of the skin" (from Latin ''cutis'' 'skin'). In [[mammal]]s, the skin is an [[organ (anatomy)|organ]] of the [[integumentary system]] made up of multiple layers of [[ectodermal]] [[tissue (biology)|tissue]] and guards the underlying [[muscle]]s, [[bone]]s, [[ligament]]s, and [[internal organs]]. Skin of a different nature exists in [[amphibian]]s, [[reptile]]s, and [[bird]]s.<ref>{{cite journal |last1=Alibardi |first1=Lorenzo |title=Adaptation to the land: The skin of reptiles in comparison to that of amphibians and endotherm amniotes |journal=Journal of Experimental Zoology |date=15 August 2003 |volume=298B |issue=1 |pages=12–41 |doi=10.1002/jez.b.24 |pmid=12949767 |bibcode=2003JEZB..298...12A }}</ref> Skin (including cutaneous and subcutaneous tissues) plays crucial roles in formation, structure, and function of extraskeletal apparatus such as horns of bovids (e.g., cattle) and rhinos, cervids' antlers, giraffids' ossicones, armadillos' osteoderm, and [[os penis]]/[[os clitoris]].<ref name=":2">{{cite journal |last1=Nasoori |first1=Alireza |title=Formation, structure, and function of extra-skeletal bones in mammals |journal=Biological Reviews |date=August 2020 |volume=95 |issue=4 |pages=986–1019 |doi=10.1111/brv.12597 |pmid=32338826 |s2cid=216556342 }}</ref>

All mammals have some hair on their skin, even [[marine mammal]]s like [[whale]]s, [[dolphin]]s, and [[porpoise]]s that appear to be hairless.
The skin interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the [[Body (biology)|body]] against [[pathogen]]s<ref name="Proksch">{{cite journal | vauthors = Proksch E, Brandner JM, Jensen JM | year = 2008 | title = The skin: an indispensable barrier | journal = Exp Dermatol | volume = 17 | issue = 12| pages = 1063–1072 | pmid = 19043850 | doi=10.1111/j.1600-0625.2008.00786.x| s2cid = 31353914 }}</ref> and excessive water loss.<ref name="Madison">{{cite journal |last1=Madison |first1=Kathi C. |title=Barrier Function of the Skin: 'La Raison d'Être' of the Epidermis |journal=Journal of Investigative Dermatology |date=August 2003 |volume=121 |issue=2 |pages=231–241 |doi=10.1046/j.1523-1747.2003.12359.x |pmid=12880413 |doi-access=free }}</ref> Its other functions are [[thermal insulation|insulation]], [[Thermoregulation|temperature regulation]], sensation, and the production of [[vitamin D]] folates.  Severely damaged skin may heal by forming [[scar|scar tissue]]. This is sometimes discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans, for example, the skin located under the eyes and around the eyelids is the thinnest skin on the body at 0.5&nbsp;mm thick and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is the thickest skin on the body at 4&nbsp;mm thick. The speed and quality of wound healing in skin is promoted by [[estrogen]].<ref>{{cite journal |last1=Thornton |first1=M. J. |title=The biological actions of estrogens on skin: Estrogens and skin |journal=Experimental Dermatology |date=December 2002 |volume=11 |issue=6 |pages=487–502 |doi=10.1034/j.1600-0625.2002.110601.x |pmid=12473056 |doi-access=free }}</ref><ref>{{cite journal |last1=Ashcroft |first1=Gillian S. |last2=Greenwell-Wild |first2=Teresa |last3=Horan |first3=Michael A. |last4=Wahl |first4=Sharon M. |last5=Ferguson |first5=Mark W.J. |title=Topical Estrogen Accelerates Cutaneous Wound Healing in Aged Humans Associated with an Altered Inflammatory Response |journal=The American Journal of Pathology |date=October 1999 |volume=155 |issue=4 |pages=1137–1146 |doi=10.1016/S0002-9440(10)65217-0 |pmid=10514397 |pmc=1867002 }}</ref><ref>{{cite journal |last1=Oh |first1=Desiree May |last2=Phillips |first2=Tania J. |title=Sex Hormones and Wound Healing |journal=Wounds |date=2006 |volume=18 |issue=1 |pages=8–18 |url=https://www.hmpgloballearningnetwork.com/site/wounds/article/5190 }}</ref>

[[Fur]] is dense hair.<ref>{{cite web|url=http://www.thefreedictionary.com/fur|title=fur|access-date=4 March 2017|via=The Free Dictionary|archive-url=https://web.archive.org/web/20170303230615/http://www.thefreedictionary.com/fur|archive-date=3 March 2017|url-status=live}}</ref> Primarily, fur augments the insulation the skin provides but can also serve as a [[Secondary sex characteristic|secondary sexual characteristic]] or as [[camouflage]]. On some animals, the skin is very hard and thick and can be processed to create [[leather]]. [[Reptile]]s and most [[fish]] have hard protective scales on their skin for protection, and [[bird]]s have hard [[feather]]s, all made of tough [[beta-keratin]]s. [[Amphibian]] skin is not a strong barrier, especially regarding the passage of chemicals via skin, and is often subject to [[osmosis]] and diffusive forces. For example, a [[frog]] sitting in an [[anesthetic]] solution would be sedated quickly as the chemical diffuses through its skin. [[Amphibian]] skin plays key roles in everyday survival and their ability to exploit a wide range of habitats and ecological conditions.<ref name=clarke>{{cite journal |last1=Clarke |first1=B. T. |title=The natural history of amphibian skin secretions, their normal functioning and potential medical applications |journal=Biological Reviews of the Cambridge Philosophical Society |date=August 1997 |volume=72 |issue=3 |pages=365–379 |doi=10.1111/j.1469-185X.1997.tb00018.x |pmid=9336100 }}</ref>

On 11 January 2024, biologists reported the discovery of the oldest known skin, fossilized about 289 million years ago, and possibly the skin from an ancient reptile.<ref name="NYT-20240111kg">{{cite news |last=Golembiewski |first=Kate |title=Scaly Fossil Is the Oldest-Known Piece of Skin - The specimen came from a 289 million-year-old fossil deposit and might offer clues to how skin evolved. |url=https://www.nytimes.com/2024/01/11/science/fossil-skin-oldest-lizard.html |date=11 January 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240111171346/https://www.nytimes.com/2024/01/11/science/fossil-skin-oldest-lizard.html |archivedate=11 January 2024 |accessdate=12 January 2024 }}</ref><ref name="CB-2-240111">{{cite journal |last1=Mooney |first1=Ethan D. |last2=Maho |first2=Tea |last3=Philp |first3=R. Paul |last4=Bevitt |first4=Joseph J. |last5=Reisz |first5=Robert R. |title=Paleozoic cave system preserves oldest-known evidence of amniote skin |journal=Current Biology |date=January 2024 |volume=34 |issue=2 |pages=417–426.e4 |doi=10.1016/j.cub.2023.12.008 |pmid=38215745 |bibcode=2024CBio...34E.417M }}</ref>

== Etymology ==
The word skin originally only referred to dressed and tanned animal hide and the usual word for human skin was hide. Skin is a borrowing from [[Old Norse]] {{lang|non|skinn}} "animal hide, fur", ultimately from the [[Proto-Indo-European]] root *sek-, meaning "to cut" (probably a reference to the fact that in those times animal hide was commonly cut off to be used as garment).<ref>{{Cite web|url=https://www.etymonline.com/word/skin#etymonline_v_23612|title = Skin &#124; Origin and meaning of skin by Online Etymology Dictionary}}</ref>

==Structure in mammals==
{{anchor|Skin layers|Layers|Layer}} <!-- required for incoming redirect-->
{{Infobox anatomy
| Name        = Dermis
| Latin       =
| Image       = Gray942.png
| Caption     = The distribution of the blood vessels in the skin of the sole of the foot. (Corium – TA alternate term for dermis – is labeled at upper right.)
| Width       =
| Image2      = Gray940.png
| Caption2    = A diagrammatic sectional view of the skin (''click on image to magnify''). (Dermis labeled at center right.)
| Precursor   =
| System      =
| Artery      =
| Vein        =
| Nerve       =
| Lymph       =
}}

Mammalian skin is composed of two primary layers:

* The ''[[epidermis]]'', which provides waterproofing and serves as a barrier to infection.
* The ''[[dermis]]'', which serves as a location for the [[appendages]] of skin.

===Epidermis===
{{main|Epidermis}}

The epidermis is composed of the outermost layers of the skin. It forms a protective barrier over the body's surface, responsible for keeping water in the body and preventing [[pathogens]] from entering, and is a stratified squamous [[epithelium]],<ref name="Rooks">{{cite book
 | last1=McGrath |first1=J.A. |last2=Eady|first2=R.A. |last3=Pope|first3=F.M.
 | title=Rook's Textbook of Dermatology
 | year=2004 | edition=7th
 | publisher=Blackwell Publishing | isbn=978-0-632-06429-8
 | pages=3.1–3.6
}}</ref> composed of [[cell division|proliferating]] basal and [[cell differentiation|differentiated]] suprabasal [[keratinocyte]]s.

[[Keratinocyte]]s are the major [[cell (biology)|cells]], constituting 95% of the [[epidermis (skin)|epidermis]],<ref name="Rooks" /> while [[Merkel cell]]s,  [[melanocyte]]s and [[Langerhans cell]]s are also present. The [[epidermis (skin)|epidermis]] can be further subdivided into the following ''strata'' or layers (beginning with the outermost layer):<ref>{{cite book|url=https://openstax.org/details/books/anatomy-and-physiology-2e |title=Anatomy and Physiology 2e |publisher=OpenStax |page=164 |last1=Betts |first1=J. Gordon |display-authors=etal |year=2022 |isbn=978-1-711494-06-7}}</ref>
* [[Stratum corneum]]
* [[Stratum lucidum]] (only in [[Hand#Human anatomy|palms]] and [[sole (foot)|soles]])
* [[Stratum granulosum]]
* [[Stratum spinosum]]
* [[Stratum basale]] (also called the ''stratum germinativum'')
[[Keratinocyte]]s in the [[stratum basale]] proliferate through [[mitosis]] and the daughter [[cell (biology)|cells]] move up the strata changing shape and composition as they undergo multiple stages of [[cell differentiation]] to eventually become anucleated. During that process, [[keratinocyte]]s will become highly organized, forming [[cell junction|cellular junctions]] ([[desmosomes]]) between each other and secreting [[keratin]] [[proteins]] and [[lipids]] which contribute to the formation of an [[extracellular matrix]] and provide mechanical [[strength of materials|strength]] to the skin.<ref name="Pmid">{{cite journal | pmid=19333614 | year=2009 | last1=Breitkreutz | first1=D | last2=Mirancea | first2=N | last3=Nischt | first3=R | title=Basement membranes in skin: Unique matrix structures with diverse functions? | volume=132 | issue=1 | pages=1–10 | doi=10.1007/s00418-009-0586-0 | journal=Histochemistry and Cell Biology| s2cid=21633122 }}</ref> [[Keratinocyte]]s from the [[stratum corneum]] are eventually shed from the surface ([[desquamation]]).

The [[epidermis (skin)|epidermis]] contains no [[blood vessel]]s, and [[cell (biology)|cells]] in the deepest layers are nourished by diffusion from [[blood]] [[capillaries]] extending to the upper layers of the [[dermis]].

===Basement membrane===
{{main|Basement membrane}}

The [[epidermis]] and [[dermis]] are separated by a thin sheet of [[fibers]] called the [[basement membrane]], which is made through the action of both [[tissue (biology)|tissues]].
The basement membrane controls the traffic of the [[cell (biology)|cells]] and [[molecules]] between the dermis and epidermis but also serves, through the binding of a variety of [[cytokine]]s and [[growth factors]], as a reservoir for their controlled release during [[physiological]] remodeling or repair processes.<ref>{{cite journal |last1=Iozzo |first1=Renato V. |title=Basement membrane proteoglycans: from cellar to ceiling |journal=Nature Reviews Molecular Cell Biology |date=August 2005 |volume=6 |issue=8 |pages=646–656 |doi=10.1038/nrm1702 |pmid=16064139 |s2cid=22151754 }}</ref>

===Dermis===
{{main|Dermis}}

The '''dermis''' is the layer of skin beneath the [[epidermis (skin)|epidermis]] that consists of [[connective tissue]] and cushions the body from [[stress (biology)|stress]] and strain. The [[dermis]] provides tensile [[strength of materials|strength]] and [[elasticity (physics)|elasticity]] to the skin through an [[extracellular matrix]] composed of [[collagen fibrils]], [[microfibrils]], and [[elastic fibers]], embedded in [[hyaluronan]] and [[proteoglycan]]s.<ref name="Pmid" /> Skin proteoglycans  are varied and have very specific locations.<ref>{{cite journal |last1=Smith |first1=Margaret Mary |last2=Melrose |first2=James |title=Proteoglycans in Normal and Healing Skin |journal=Advances in Wound Care |date=March 2015 |volume=4 |issue=3 |pages=152–173 |doi=10.1089/wound.2013.0464 |pmid=25785238 |pmc=4352701 }}</ref> For example, [[hyaluronan]], [[versican]] and [[decorin]] are present throughout the dermis and [[epidermis]] [[extracellular matrix]], whereas [[biglycan]] and [[perlecan]] are only found in the epidermis.

It harbors many [[mechanoreceptor]]s (nerve endings) that provide the sense of [[touch]] and heat through [[nociceptor]]s and [[thermoreceptor]]s. It also contains the [[hair follicle]]s, [[sweat gland]]s, [[sebaceous gland]]s, [[apocrine glands]], [[lymphatic vessel]]s and [[blood vessel]]s. The [[blood vessel]]s in the [[dermis]] provide nourishment and waste removal from its own [[cell (biology)|cells]] as well as for the [[epidermis (skin)|epidermis]].

Dermis and subcutaneous tissues are thought to contain germinative cells involved in formation of horns, osteoderm, and other extra-skeletal apparatus in mammals.<ref name=":2" />

The [[dermis]] is tightly connected to the [[epidermis (skin)|epidermis]] through a [[basement membrane]] and is structurally divided into two areas: a superficial area adjacent to the epidermis, called the ''papillary region'', and a deep thicker area known as the ''reticular region''.

====Papillary region====
The papillary region is composed of loose [[areolar connective tissue]]. This is named for its fingerlike projections called ''papillae'' that extend toward the [[epidermis (skin)|epidermis]]. The papillae provide the [[dermis]] with a "bumpy" surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.

====Reticular region====
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular [[connective tissue]] and receives its name from the dense concentration of [[collagen]]ous, [[Elastic fibers|elastic]], and [[Reticular fiber|reticular]] fibers that weave throughout it. These [[protein]] fibers give the [[dermis]] its properties of [[strength of materials|strength]], [[extensibility]], and [[elasticity (physics)|elasticity]].
Also located within the reticular region are the [[roots of the hair]], [[sweat gland]]s, [[sebaceous gland]]s, [[Cutaneous receptor|receptors]], [[nail (anatomy)|nails]], and [[blood vessel]]s.

===Subcutaneous tissue===
{{main|Subcutaneous tissue}}

The [[subcutaneous tissue]] (also hypodermis)  is not part of the skin, and lies below the [[dermis]]. Its purpose is to attach the skin to underlying [[bone]] and [[muscle]] as well as supplying it with [[blood vessel]]s and [[nerve]]s. It consists of loose [[connective tissue]] and [[elastin]]. The main [[cell (biology)|cell]] types are [[fibroblast]]s, [[macrophage]]s and [[adipocyte]]s (the subcutaneous tissue contains 50% of [[body fat]]). [[Fat]] serves as padding and insulation for the body.

[[Microorganisms]] like ''[[Staphylococcus epidermidis]]'' colonize the skin surface. The density of [[skin flora]] depends on region of the skin. The disinfected skin surface gets recolonized from [[bacteria]] residing in the deeper areas of the [[hair follicle]], [[gastrointestinal tract|gut]] and [[urogenital]] openings.

===Detailed cross section===
{{wide image|Skin_layers.svg|800px|Skin layers, of both the hairy and hairless skin}}

==Structure in fish, amphibians, birds, and reptiles==
===Fish===
{{Further|Fish scale}}
{{One source section|date=August 2021}}

The epidermis of [[fish]] and of most [[amphibian]]s consists entirely of live [[cell (biology)|cells]], with only minimal quantities of [[keratin]] in the cells of the superficial layer.<ref name="auto">{{Cite journal |last1=Varga |first1=Joseph F. A. |last2=Bui-Marinos |first2=Maxwell P. |last3=Katzenback |first3=Barbara A. |date=2019 |title=Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens |journal=Frontiers in Immunology |volume=9 |page=3128 |doi=10.3389/fimmu.2018.03128 |pmid=30692997 |pmc=6339944 |issn=1664-3224|doi-access=free }}</ref> It is generally permeable, and in the case of many [[amphibians]], may actually be a major respiratory organ.<ref>{{Cite journal |last1=Ferrie |first1=Gina M. |last2=Alford |first2=Vance C. |last3=Atkinson |first3=Jim |last4=Baitchman |first4=Eric |last5=Barber |first5=Diane |last6=Blaner |first6=William S. |last7=Crawshaw |first7=Graham |last8=Daneault |first8=Andy |last9=Dierenfeld |first9=Ellen |last10=Finke |first10=Mark |last11=Fleming |first11=Greg |date=2014 |title=Nutrition and Health in Amphibian Husbandry |journal=Zoo Biology |volume=33 |issue=6 |pages=485–501 |doi=10.1002/zoo.21180 |issn=0733-3188 |pmc=4685711 |pmid=25296396}}</ref> The [[dermis]] of [[bony fish]] typically contains relatively little of the [[connective tissue]] found in [[tetrapod]]s. <ref name="auto"/> Instead, in most species, it is largely replaced by solid, protective bony [[scale (zoology)|scales]].<ref>{{Cite web |last=Fisheries |first=NOAA |date=2022-05-03 |title=Fun Facts About Shocking Sharks {{!}} NOAA Fisheries |url=https://www.fisheries.noaa.gov/national/outreach-and-education/fun-facts-about-shocking-sharks |access-date=2022-05-11 |website=NOAA |language=en}}</ref> Apart from some particularly large dermal bones that form parts of the [[skull]], these [[scale (zoology)|scales]] are lost in [[tetrapods]], although many [[reptile]]s do have [[scale (zoology)|scales]] of a different kind, as do [[pangolin]]s.<ref>{{Cite web |title=Pangolin Fact Sheet {{!}} Blog {{!}} Nature {{!}} PBS |url=https://www.pbs.org/wnet/nature/blog/pangolin-fact-sheet/ |access-date=2022-05-11 |website=Nature |date=25 March 2020 |language=en-US}}</ref> [[Cartilaginous fish]] have numerous tooth-like [[Dermal denticle|denticles]] embedded in their skin, in place of true [[scale (zoology)|scales]].<ref>{{Cite journal |title=Shark and Ray Workbook 3-5 update 8-31 |url=https://www.floridaocean.org/sites/default/files/images/Shark%20and%20Ray%20Workbook%203-5%20update%208-31.pdf |journal=Florida Oceanographic Society}}</ref>

[[Sweat gland]]s and [[sebaceous gland]]s are both unique to [[mammals]], but other types of skin gland are found in other [[vertebrates]].{{fact|date=September 2024}} [[Fish]] typically have a numerous individual [[mucus]]-secreting skin [[cell (biology)|cells]] that aid in insulation and protection, but may also have [[poison]] [[glands]], [[photophore]]s, or [[cell (biology)|cells]] that produce a more watery, [[serous]] fluid. In [[amphibians]], the mucous cells are gathered together to form sac-like [[glands]]. Most living [[amphibians]] also possess ''granular [[glands]]'' in the skin, that secrete irritating or toxic compounds.<ref name=VB/>

Although [[melanin]] is found in the skin of many species, in the [[reptile]]s, the [[amphibians]], and [[fish]], the [[epidermis (skin)|epidermis]] is often relatively colorless. Instead, the color of the skin is largely due to [[chromatophore]]s in the [[dermis]], which, in addition to melanin, may contain [[guanine]] or [[carotenoid]] [[pigments]]. Many species, such as [[chameleon]]s and [[flounder]]s may be able to change the color of their skin by adjusting the relative size of their [[chromatophores]].<ref name=VB/>

=== Amphibians ===
{{Further|Amphibian#Skin}}

==== Overview ====
[[File:FrogSkin.png|thumb|616x616px|'''Frog gland anatomy–''' A: Mucous gland (alveolus), B: Chromophore, C: Granular gland (alveolus), D: Connective tissue, E: Stratum corneum, F: Transition zone (intercalary region), G: Epidermis (where the duct resides), H: Dermis]]

Amphibians possess two types of [[gland]]s, [[Mucous gland|mucous]] and granular (serous). Both of these glands are part of the [[integument]] and thus considered [[cutaneous]]. Mucous and granular glands are both divided into three different sections which all connect to structure the gland as a whole. The three individual parts of the gland are the duct, the intercalary region, and lastly the alveolar gland (sac). Structurally, the duct is derived via [[keratinocyte]]s and passes through to the surface of the [[Epidermis|epidermal]] or outer skin layer thus allowing external secretions of the body. The gland alveolus is a sac-shaped structure that is found on the bottom or base region of the granular gland. The cells in this sac specialize in secretion. Between the alveolar gland and the duct is the intercalary system which can be summed up as a transitional region connecting the duct to the grand alveolar beneath the epidermal skin layer. In general, granular glands are larger in size than the mucous glands, which are greater in number.<ref name=":0">{{cite journal |last1=Toledo |first1=R.C. |last2=Jared |first2=C. |title=Cutaneous granular glands and amphibian venoms |journal=Comparative Biochemistry and Physiology Part A: Physiology |date=May 1995 |volume=111 |issue=1 |pages=1–29 |doi=10.1016/0300-9629(95)98515-I }}</ref>

==== Granular glands ====
Granular glands can be identified as [[venom]]ous and often differ in the type of toxin as well as the concentrations of secretions across various orders and species within the amphibians. They are located in clusters differing in concentration depending on [[amphibian]] taxa. The toxins can be fatal to most vertebrates or have no effect against others. These glands are alveolar meaning they structurally have little sacs in which venom is produced and held before it is secreted upon defensive behaviors.<ref name=":0" />

Structurally, the ducts of the granular gland initially maintain a cylindrical shape. When the ducts mature and fill with fluid, the base of the ducts become swollen due to the pressure from the inside. This causes the epidermal layer to form a pit like opening on the surface of the duct in which the inner fluid will be secreted in an upwards fashion.<ref name=":1">{{cite journal |last1=Dawson |first1=A. B. |title=The integument of necturus maculosus |journal=Journal of Morphology |date=December 1920 |volume=34 |issue=3 |pages=486–589 |doi=10.1002/jmor.1050340303 |s2cid=83534922 |url=https://books.google.com/books?id=UklQAQAAMAAJ&pg=PA487|url-access=subscription }}</ref>

The intercalary region of granular glands is more developed and mature in comparison with mucous glands. This region resides as a ring of cells surrounding the basal portion of the duct which are argued to have an [[ectoderm]]al muscular nature due to their influence over the lumen (space inside the tube) of the duct with dilation and constriction functions during secretions. The cells are found radially around the duct and provide a distinct attachment site for muscle fibers around the gland's body.<ref name=":1" />

The gland alveolus is a sac that is divided into three specific regions/layers. The outer layer or tunica fibrosa is composed of densely packed connective-tissue which connects with fibers from the spongy intermediate layer where elastic fibers, as well as nerves, reside. The nerves send signals to the muscles as well as the epithelial layers. Lastly, the epithelium or tunica propria encloses the gland.<ref name=":1" />

==== Mucous glands ====
[[Mucous gland]]s are non-venomous and offer a different functionality for amphibians than granular. Mucous glands cover the entire surface area of the amphibian body and specialize in keeping the body lubricated. There are many other functions of the mucous glands such as controlling the pH, thermoregulation, adhesive properties to the environment, anti-predator behaviors (slimy to the grasp), chemical communication, even anti-bacterial/viral properties for protection against pathogens.<ref name=":0" />

The ducts of the mucous gland appear as cylindrical vertical tubes that break through the epidermal layer to the surface of the skin. The cells lining the inside of the ducts are oriented with their longitudinal axis forming 90-degree angles surrounding the duct in a helical fashion.<ref name=":1" />

Intercalary cells react identically to those of granular glands but on a smaller scale. Among the amphibians, there are taxa which contain a modified intercalary region (depending on the function of the glands), yet the majority share the same structure.<ref name=":1" />

The alveolar or mucous glands are much more simple and only consist of an epithelium layer as well as connective tissue which forms a cover over the gland. This gland lacks a tunica propria and appears to have delicate and intricate fibers which pass over the gland's muscle and epithelial layers.<ref name=":1" />

===Birds and reptiles===
{{Main|Reptile scales}}
{{One source section|date=August 2021}}

The [[epidermis (skin)|epidermis]] of [[bird]]s and [[reptile]]s is closer to that of [[mammals]], with a layer of dead keratin-filled [[cell (biology)|cells]] at the surface, to help reduce water loss. A similar pattern is also seen in some of the more terrestrial [[amphibians]] such as [[toad]]s. In these animals, there is no clear [[cell differentiation|differentiation]] of the [[epidermis (skin)|epidermis]] into distinct layers, as occurs in [[human]]s, with the change in [[cell (biology)|cell]] type being relatively gradual. The mammalian [[epidermis (skin)|epidermis]] always possesses at least a [[stratum germinativum]] and [[stratum corneum]], but the other intermediate layers found in [[human]]s are not always distinguishable.
[[Hair]] is a distinctive feature of mammalian skin, while [[feather]]s are (at least among living species) similarly unique to [[bird]]s.<ref name=VB>{{cite book |author=Romer, Alfred Sherwood|author2=Parsons, Thomas S.|year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia|pages= 129–145|isbn= 978-0-03-910284-5}}</ref>

[[Bird]]s and [[reptile]]s have relatively few skin [[gland]]s, although there may be a few structures for specific purposes, such as [[pheromone]]-secreting [[cell (biology)|cells]] in some [[reptile]]s, or the [[uropygial gland]] of most birds.<ref name=VB/>

==Development==
{{One source section|date=August 2021}}

Cutaneous structures arise from the [[Epidermis (skin)|epidermis]] and include a variety of features such as hair, feathers, claws and nails. During embryogenesis, the epidermis splits into two layers: the periderm (which is lost) and the [[Stratum germinativum|basal layer]].  The basal layer is a [[stem cell]] layer and through asymmetrical divisions, becomes the source of skin cells throughout life. It is maintained as a stem cell layer through an [[autocrine]] signal, [[TGF alpha]], and through [[paracrine]] signaling from FGF7 ([[keratinocyte growth factor]]) produced by the dermis below the basal cells. In mice, over-expression of these factors leads to an overproduction of [[Stratum granulosum|granular cells]] and thick skin.<ref>{{Cite journal |last1=Vassar |first1=Robert |last2=Fuchs |first2=Elaine |date=February 28, 1991 |title=Transgenic mice provide new insights into the role of TGF- during epidermal development and differentiation |url=http://genesdev.cshlp.org/content/5/5/714.full.pdf |journal=Genes & Development |volume=1991 |issue=5 |pages=714–727|doi=10.1101/gad.5.5.714 |pmid=1709129 }}</ref><ref>{{cite journal |last1=Vassar |first1=R |last2=Fuchs |first2=E |title=Transgenic mice provide new insights into the role of TGF-alpha during epidermal development and differentiation |journal=Genes Dev |date=1 May 1991 |volume=5 |issue=5 |pages=714–727 |doi=10.1101/gad.5.5.714 |pmid=1709129 |doi-access=free }}</ref>

{{Cn span|Hair and feathers are formed in a regular pattern and it is believed to be the result of a reaction-diffusion system. This [[reaction-diffusion system]] combines an activator, [[Sonic hedgehog]], with an inhibitor, BMP4 or BMP2, to form clusters of cells in a regular pattern. Sonic hedgehog-expressing epidermal cells induce the condensation of cells in the [[mesoderm]]. The clusters of mesodermal cells signal back to the epidermis to form the appropriate structure for that position. BMP signals from the epidermis inhibit the formation of placodes in nearby ectoderm.|date=December 2017}}

It is believed that the mesoderm defines the pattern. The epidermis instructs the mesodermal cells to condense and then the mesoderm instructs the epidermis of what structure to make through a series of reciprocal inductions. Transplantation experiments involving frog and newt epidermis indicated that the mesodermal signals are conserved between species but the epidermal response is species-specific meaning that the mesoderm instructs the epidermis of its position and the epidermis uses this information to make a specific structure.<ref>{{cite book |last1=Gilbert |first1=Scott F. |title=Developmental Biology. 6th edition |date=2000 |publisher=Sinauer Associates |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK9993/ |chapter=Induction and Competence }}</ref>

==Functions==
Skin performs the following functions:

# Protection: an anatomical barrier from [[pathogens]] and damage between the internal and external  [[natural environment|environment]] in bodily defense. (See [[Skin absorption]].) [[Langerhans cell]]s in the skin are part of the [[adaptive immune system]].<ref name="Proksch"/><ref name="Madison"/>
# [[Wikt:sensation|Sensation]]: contains a variety of [[nerve endings]] that jump to [[thermoreceptor|heat and cold]], [[touch]], [[pressure]], [[vibration]], and [[tissue (biology)|tissue]] [[injury]] (see [[somatosensory system]] and [[haptic perception]]).
# Thermoregulation: [[Eccrine]] ([[sweat]]) glands and dilated blood vessels (increased superficial [[perfusion]]) aid heat loss, while constricted [[blood vessel|vessels]] greatly reduce cutaneous [[blood flow]] and conserve heat. [[Erector pili muscle]]s in mammals adjust the angle of hair shafts to change the degree of insulation provided by hair or [[fur]].
# Control of [[evaporation]]: the skin provides a relatively dry and semi-impermeable barrier to reduce fluid loss.<ref name="Madison"/>
# Storage and [[biosynthesis|synthesis]]: acts as a storage center for [[lipid]]s and water
# [[Skin absorption|Absorption through the skin]]: [[Oxygen]], [[nitrogen]] and [[carbon dioxide]] can diffuse into the [[epidermis (skin)|epidermis]] in small amounts; some animals use their skin as their sole [[respiration organ]] (in [[human]]s, the [[cell (biology)|cells]] comprising the outermost 0.25–0.40&nbsp;mm of the skin are "almost exclusively supplied by external oxygen", although the "contribution to total [[respiration (physiology)|respiration]] is negligible")<ref>{{cite journal |last1=Stücker |first1=M. |last2=Struk |first2=A. |last3=Altmeyer |first3=P. |last4=Herde |first4=M. |last5=Baumgärtl |first5=H. |last6=Lübbers |first6=D. W. |title=The cutaneous uptake of atmospheric oxygen contributes significantly to the oxygen supply of human dermis and epidermis |journal=The Journal of Physiology |date=February 2002 |volume=538 |issue=3 |pages=985–994 |doi=10.1113/jphysiol.2001.013067 |pmid=11826181 |pmc=2290093 }}</ref> Some [[Topical medication|medications are absorbed through the skin]].
# Water resistance: The skin acts as a water resistant barrier so essential [[nutrients]] aren't washed out of the body. The nutrients and oils that help hydrate the skin are covered by the most outer skin layer, the [[Epidermis (skin)|epidermis]]. This is helped in part by the sebaceous glands that release [[sebum]], an oily liquid. Water itself will not cause the elimination of oils on the skin, because the oils residing in our dermis flow and would be affected by water without the epidermis.<ref>{{cite book|last=McCracken|first=Thomas|title=New Atlas of Human Anatomy|year=2000|publisher=Metro Books|location=China|isbn=978-1-58663-097-3|pages=1–240}}</ref>
# [[Camouflage]], whether the skin is naked or covered in fur, scales, or feathers, skin structures provide protective coloration and patterns that help to conceal animals from predators or prey.<ref>{{cite magazine |title=Camouflage |url=http://www.nationalgeographic.org/encyclopedia/camouflage/ |magazine=National Geographic |access-date=27 February 2017 |date=2011-08-25 |archive-url=https://web.archive.org/web/20170227232527/http://www.nationalgeographic.org/encyclopedia/camouflage/ |archive-date=27 February 2017 |url-status=live }}</ref>

===Mechanics===
{{Main|Soft tissue}}

Skin is a soft tissue and exhibits key mechanical behaviors of these tissues.  The most pronounced feature is the J-curve stress strain response, in which a region of large strain and minimal stress exists and corresponds to the microstructural straightening and reorientation of collagen fibrils.<ref>{{Cite journal |last=Sherman |first=Vincent R. |title=The materials science of collagen |journal=Journal of the Mechanical Behavior of Biomedical Materials |volume=52 |pages=22–50 |year=2015 |doi=10.1016/j.jmbbm.2015.05.023 |pmid=26144973|doi-access=free }}</ref> In some cases the intact skin is prestreched, like wetsuits around the diver's body, and in other cases the intact skin is under compression. Small circular holes punched on the skin may widen or close into ellipses, or shrink and remain circular, depending on preexisting stresses.<ref>{{cite journal |last1=Bush |first1=James A. |last2=Ferguson |first2=Mark W.J. |last3=Mason |first3=Tracey |last4=McGrouther |first4=D. Angus |title=Skin tension or skin compression? Small circular wounds are likely to shrink, not gape |journal=Journal of Plastic, Reconstructive & Aesthetic Surgery |date=May 2008 |volume=61 |issue=5 |pages=529–534 |doi=10.1016/j.bjps.2007.06.004 |pmid=17652049 }}</ref>

===Aging===
Tissue [[homeostasis]] generally declines with age, in part because [[stem cell|stem]]/progenitor cells fail to self-renew or [[cellular differentiation|differentiate]]. Skin aging is caused in part by [[TGF-β]] by blocking the conversion of [[dermal fibroblasts]] into [[fat cells]] which provide support. Common changes in the skin as a result of aging range from [[wrinkle]]s, discoloration, and skin laxity, but can manifest in more severe forms such as skin malignancies.<ref name=":3">{{cite journal |last1=Hashizume |first1=Hideo |title=Skin Aging and Dry Skin |journal=The Journal of Dermatology |date=August 2004 |volume=31 |issue=8 |pages=603–609 |doi=10.1111/j.1346-8138.2004.tb00565.x |pmid=15492432 |s2cid=44823023 }}</ref><ref name=":4">{{cite journal |last1=Rabe |first1=Jessica H. |last2=Mamelak |first2=Adam J. |last3=McElgunn |first3=Patrick J.S. |last4=Morison |first4=Warwick L. |last5=Sauder |first5=Daniel N. |title=Photoaging: Mechanisms and repair |journal=Journal of the American Academy of Dermatology |date=July 2006 |volume=55 |issue=1 |pages=1–19 |doi=10.1016/j.jaad.2005.05.010 |pmid=16781287 }}</ref> Moreover, these factors may be worsened by sun exposure in a process known as [[photoaging]].<ref name=":4" />

==See also==
{{Portal|Anatomy}}
{{Div col|small=yes}}
* [[Cutaneous reflex in human locomotion]]
* [[Cutaneous respiration]] – gas exchange conducted through skin
* [[Moult]]
* [[Role of skin in locomotion]]
* [[Skinning]]
{{Div col end}}

==References==
{{Reflist}}

==External links==
* {{Wiktionary-inline}}

{{Integumentary physiology}}
{{Development of integument}}
{{Authority control}}

[[Category:Skin| ]]
[[Category:Animal anatomy]]
[[Category:Leathermaking]]
[[Category:Organs (anatomy)]]
[[Category:Skin physiology]]
[[Category:Soft tissue]]